A series of very complex time and temperature dependent chemical reactions accompany the burning of wood, making it difficult to supply the correct amount of air and to control the output to match a particular heating load. This difficulty in carburetion and control of wood fuel combustion is compounded by complexities of ignition. The pyrolysis gases generated from heating wood have ignition temperatures over a range from 725 degrees for methanol to 1128 degrees for carbon monoxide. Since conventional wood stove and wood furnace surface temperatures do not attain this range, much of the gas distilled from wood during burning is vented up the chimney. Conventional wood stoves and furnaces therefore suffer undesirable consequences of inefficient loss of fuel energy, pollution of the atmosphere, and chimney condensation or "creosote" deposits with subsequent fire hazard.
As a piece of wood is burned, heat is transferred from the surface to the interior of the wood, with a counterflow of pyrolysis material from the interior to the surface. The kinetics of the reaction depend upon many factors including the surface to volume ratio of the wood piece, surface temperature including radiant field and convection field, wood moisture, wood species, and rate of air supply. This complexity of parameters conspires to produce considerable variation in output and performance in conventional wood stoves and furnaces.
It is therefore an object of the present invention to provide a new and improved wood furnace system design and method which maintain the combustion zone at sufficient temperature and turbulence and which maintain the pyrolysis gases generated by wood burning in the high temperature combustion zone for a sufficient time to permit substantially complete combustion of wood pyrolysis materials. According to the invention the chemical reactions accompanying the burning of wood are substantially completed in a high temperature delayed propagation zone prior to heat exchange from the end products of combustion, thereby greatly reducing inefficient loss of fuel energy, pollution of the atmosphere, and chemical condensation in the chimney.
Another object of the invention is to provide a wood stove or furnace with a confined locus of wood fuel combustion in a high temperature environment and for gravity feed of the wood fuel into the confined locus of efficient combustion by progressive burning from the bottom of the charge of wood, thereby providing a steady state burn and steady state output from the furnace system as the charge of wood fuel is consumed.
A further object of the invention is to provide a multiple use furnace and boiler unit for house and building heating, domestic hot water use, and for hot water storage derived from a safe, stable and reliable form of wood fuel combustion.